Collects 16S rRNA sequences from a large number of datasets. MetaMetaDB is a comprehensive (‘‘meta-’’) and compact database that contains collection of 16S rRNA sequences associated with diverse environments. Users can submit the 16S rRNA sequences of certain prokaryotes and thus can investigate the microbial habitability for analyzing the ecology and evolution of prokaryotes. The database provides a reverse perspective of the environments in which each prokaryotic group exists, opening the door to the investigation of ‘‘meta-metagenomics’’.